The number of older adults is expected to double by 2030. With the growth of the older population comes a parallel expectation of growth in the burden of age-related declines in physical function. Observational data show wide variations in the functional capabilities of older adults and have identified several potentially modifiable factors that influence the risk of functional decline. While the role of nutrition is well-establised in delaying or preventing chronic disease, because of the lack of large prospective studies of nutrition and diet in older adults, less is known about the role of nutrition in the maintenance of physical function. Nonetheless, increasing evidence suggests that nutritional factors are important to the maintenance of physical function. Loss of lean mass with aging, and the associated declines in strength, are well documented and partially account for aging-related loss of physical function. Inadequate dietary protein has been suggested as a factor that may contribute to the declines in lean mass, strength, and physical function that occurs with age. National data suggest that a significant proportion of older adults consume inadequate protein. Furthermore, current dietary protein requirements are based predominately on short-term nitrogen balance studies in young adults and may underestimate the intakes needed to optimally preserve lean mass, strength, or physical function in older adults. We propose to examine the association between dietary protein intake and change in strength and both objectively and subjectively measured physical function over 6 years of follow-up using data from the Health ABC study, a prospective cohort of 3,075 black and white men and women in their seventies who have been followed annually since 1997-98. The following specific aims will be addressed: 1) to determine whether dietary protein intake predicts change in strength and physical performance~ and 2) to determine whether dietary protein intake predicts incident mobility limitation. We hypothesize that participants with higher intakes of protein will have smaller declines in strength and physical performance and a decreased risk of incident mobility limitation over 6 years of follow-up compared to those with lower intakes of protein. In exploratory analyses, we will also examine the association between protein source (e.g., animal vs. vegetable) and essential amino acid intake (particularly branched chain amino acids such as leonine) and change in physical function~ whether the associations between dietary protein intake and physical function are mediated through changes in lean mass~ whether the associations between protein and physical function are modified by other risk factors (e.g., physical inactivity, obesity, weight loss, inflammation, and insulin resistance)~ and whether the associations between dietary protein intake and physical function differ by race or help explain racial disparities in physical function. The proposed analyses will determine the role of dietary protein in the maintenance of physical function in older adults and inform dietary recommendations regarding the level of optimal protein intake for the maintenance of physical function in an aging population.